Titanium-nickel alloy manufacturing methods

ABSTRACT

TITANIUM AND NICKEL ARE ALLOYED BY A UNIQUE PROCESSING PROCEDURE TO FORM HIGHLY USEFUL TITANIUM-NICKEL ALLOYS SUCH AS THE WELL-KNOWN TI-55 NI AND TI-60 NI ALLOYS. THE METHODS INCLUDE BLENDING OF POWDERS OF NICKEL AND TITANIUM FOLLOWED BY PESSING AND SINTERING TO FORM PREFORMS OR FINAL SHAPES OF THE DESRIED TITANIUM-NICKEL ALLOYS.

United States Patent Office 3,700,434 Patented Oct. 24, 1972 3,700,434TITANIUM-NICKEL ALLOY MANUFACTURING METHODS Int. Cl. B221? 1/00 US. Cl.75-214 7 Claims ABSTRACT OF THE DISCLOSURE Titanium and nickel arealloyed by a unique processing procedure to form highly usefultitanium-nickel alloys such as the Well-known Ti-SS Ni and Ti-60 Nialloys. The methods include blending of powders of nickel and titaniumfollowed by pressing and sintering to form preforms or final shapes ofthe desired titaniumnickel alloys.

BACKGROUND OF THE INVENTION Titanium-nickel alloys have been developedin part by the United States government as described in Pat. No.2,174,851. Such alloys have unique properties which include unusuallyhigh hardness values, corrosion resistance, nonmagnetic properties, andhigh strength. Another highly desirable unique characteristic is thememory of titanium-nickel alloys such as Ti-55 Ni. Known applicationsfor these titanium-nickel alloys, which are known as Nitinol alloys,include noise and vibration reducing components, temperature sensingdevices, outer space and hydrospace erectible structures and cryogenicequipment.

In spite of the well-known advantages of Nitinol alloys, theircommercial use is limited due at least in great part to the high cost ofworking the alloys. The alloys are conventionally made by melting toform alloy ingots which ingots are then worked by mechanical means suchas milling and forging to final finished shapes. The alloys areextremely brittle and require extreme care in mechanical working oftennecessitating repeated heating of the material as it work hardens and asit is shaped. The expense of such working steps often makes the alloyeconomically prohibitive for use in otherwise desirable commercialapplications.

It is an object of the present invention to provide titanium-nickelalloys having good physical characteristics which alloys can be maderapidly and efliciently.

Another object of this invention is to provide methods of forming thealloys as described above in preforms and/or final shapes which minimizeconventional mechanical shaping and forming techniques.

SUMMARY OF THE INVENTION According to the invention, titanium and nickelpowdered metal particles are blended to a desired alloy composition. Theblend is then pressed directly into an as pressed preform close in shapeand dimension to the final shape desired. The shape is then sintered ina protective inert atmosphere at a temperature in the range of from16-50 F. to 1750 F. to permit alloying of the particles and formation ofa final shape desired or a preform close in shape and dimension to thefinal product desired. The resultant sintered shape requires little orno forming or machining by conventional mechanical shaping proceduresand has the desirable properties of titanium-nickel alloys made byconventional melting techniques.

Since final shapes or preforms are product directly, the brittle natureof the titanium-nickel alloys does not preclude their economical use forthe large variety of commercial applications already known and foradditional uses where costs were heretofore considered prohibitive.Costs of forming shaped articles are maintained at commercially feasiblelevels since conventional mechanical steps such as forging and millingare either eliminated or greatly reduced.

BRIEF DESCRIPTION OF PREFERRED EMBODIMENTS The powders of titanium andnickel which are used in the process of the present invention must havehigh purity and are of irregular shape. For example, titanium particlesin the powder preferably have a purity of at least 99.5% by weight withno more than 0.12% oxygen and the nickel powder preferably has a purityof at least 99.7% by weight. Irregular shaped particles are used sincethey maximize mechanical interlock of the particles. Preferably, theparticle size of both the nickel and titanium particles are meshA.S.T.M. which aids in producing homogeneous products, however, particlesizes can be higher. Titanium powder useful in this invention includesstandard commercial pure titanium particles of irregular shape. Nickelpowder useful in this invention include standard commercially purenickel particles of irregular shape.

In a first step, the selected metal powders are mixed in a conventionalpowder-metal blender to form a uniform mixture of the powders.Preferably the particle sizes of the nickel and titanium particles inthe powders are substantially uniform and are of irregular shape.

The composite blend is then cold pressed directly into an as pressedpreform or substantially the final shape desired in either conventionalmechanical or conventional isostatic pressing equipment. The pressingforms a mechanical interlock of the particles and is preferably carriedout at room temperature for a few seconds to a few minutes as isconventional when pressing powdered metals. Hot pressing below sinteringtemperatures can be used but provides no significant advantage whileincreasing cost.

The shape formed by the pressing step is then sintered in a protectiveatmosphere at a sintering temperature in the range of from 1650 F. to1750 F. and preferably 1700 F. for a period of time sufiicient to formthe alloy Without causing melting. Sintering within the temperaturerange stated is important to obtain the desired properies in theresultant alloys. The alloys formed by sintering can be furtherprocessed by additional heat treatments at temperatures above the 1750F. limit. For example, where the nickel content of the alloy isapproximately 60% by weight or higher, the sintered preforms or shapescan be heated to temperatures of about 2250" F. to increase thehomogeneous nature of the alloy and increase the density. However, it isimportant to first permit alloying of the metals during sintering beforeraising the temperature above 1750 F.

The pressing step is preferably carried out at a pressure of between30,000 to 160,000 p.s.i. with the 'lower figure insuring adequate aspressed density and the upper figure preventing die damage inconventional dies used.

The sintering step can be carried out for periods of time in the rangeof /2 hour to 20 hours and preferably 1 to 2 hours with additional timesbeyond 20 hours adding no useful benefits to the material and adding tothe cost of the process. The protective atmosphere used can be a vacuumsuch as a vacuum in the range of 1X10 torr or can comprise an inert gasatmosphere in the conventional sintering enclosure of the preform orfinal shape using helium, argon or other inert gas. In some cases, acombination of a low vacuum and flushing inert gas can be used as isknown in the art. Preferably the sintering step is carried out atatmospheric pressure or in vacuum although higher pressures can be used.

It is preferred to press to form a mechanical interlock of the titaniumand nickel particles with an as pressed density of at least 70% afterwhich sintering is used to cause alloying and increase the density ofthe sintered preform or final shape to a value of at least 80% andpreferably at least 90% of the theoretical density.

The amounts of titanium and nickel in the preform or final shape aresubstantially identical to the amounts of powders originallyincorporated. Preferably the Nitinol alloys formed by the method of thisinvention generally comprise from 38 to 46 by weight titanium and 62 to54% by weight of the alloy of nickel.

In a specific example of this invention, 54.5% by weight of irregular(acicular) particles of commercially pure nickel powder (99.7% purity)are uniformly mixed with 45.5% by weight commercially pure titaniumparticles (99.5% purity) of irregular (acicular) shape with all theparticles having a size of -100 mesh A.S.T.M. The composite blend isthen pressed into the shape of a standard tensile test blank (A.S.T.M.E8-61T) at standard room temperature (72 F.) using a pressure of 60,000p.s.i. The blank formed which has a density of approximately 86% oftheoretical density, is then sintered in a furnace at a vacuum of 1x10torr and at a temperature of 1700" F. for 5 hours after which it wasallowed to cool to room temperature in the furnace in accordance withconventional practice. The resulting sintered test blank has a densityof 91% and substantially similar physical properties as a correspondingtitanium-nickel alloy made by conventional melting techniques.

For example, the resultant alloy has an ultimate tensile strength of29,400 p.s.i., a yield strength (0.2% p.s.i.) of 22,800, an elongationof 1.8% and a percent reduction of area of 3.2 when tested by standardA.S.T.M. methods (15-8).

The resultant allo'y test blank (which is considered a final shape orpreform) has the known memory characteristics of 55 Nitinol. One sampletest blank formed as described in the above example was bowed in itsflat plane when removed from the sintering furnace. Standard test blanksare flat and have dimensions of approximately 0.225 inch width by 0.125inch thickness by 3 /2 inch length but the test blank used was uniformlybowed from a flat surface on which its ends are placed by a maximumdistance (X) of 0.047. This bow existed above the transitiontemperature. The sample was cooled in ice water and further deformed toincrease the maximum distance (X) noted to 0.058. It was then cycledbetween 170 C. and 0 C. to check the memory characteristic with theresults noted below:

Temperature: Distance X, inches 0 C. 0.057 170 C. 0.0480.049 0 C. 0.059170 C. 0.0480.04'9 0 C. 0.0580.059

4 titanium particles as starting materials, a titanium-nickel alloy isformed as a test bar having a yield strength (0.2%) of 25,000 p.s.i. anda density of 89% of theoretical density. Similar useful results areobtained when the amounts of titanium and nickel are varied within theranges given above.

While specific embodiments of the present invention have been described,many variations thereof are possible within the scope of this invention.

What is claimed is:

1. A method of forming a sintered alloy consisting essentially oftitanium and nickel, having the characteristics of corresponding alloysformed by melt ingot processes,

said method comprising,

uniformly mixing acicular particles of titanium and nickel in an amountof from 38 to 46% by weight titanium and 62 to 54% by weight nickel,

consolidating said particles by pressing at a pressure in the range offrom 30,000 to 160,000 p.s.i.,

and then sintering at a temperature in the range of from 1650 F. to 1750F. to form an alloy in a usable shape.

2. A method in accordance with the method of claim 1 wherein saidparticles of titanium and nickel have a size below 100 mesh.

3. A method in accordance with the method of claim 1 wherein saidparticles are first pressed into a mechanically interlocked shape bysaid consolidating and then sintered in a protective atmosphere to formsaid alloy.

4. A method in accordance with the method of claim 3 wherein saidatmosphere is a vacuum.

5. A method in accordance with the method of claim 1 wherein saidsintering is carried out for a period of from A2 to 20 hours.

6. A method in accordance with the method of claim 5 wherein the aspressed density is at least of theoretical density and the finalsintered density is at least 80% of theoretical density.

'7. A method in accordance with the method of claim 3 wherein saidprotective atmosphere is an inert gas.

References Cited UNITED STATES PATENTS 2,163,224 6/1939 Alexander -225 X3,335,002 8/1967 Clarke 75-214 X 2,839,819 6/1958 Platte 75225 X2,227,176 12/1940 Berghaus et al. 75200 3,164,466 1/1965 Yasuda et a1.75200 OTHER REFERENCES Fundamental Principles of Powder Metallurgy, W.D. Jones, p. xi, Edward Arnold (Publishers) Ltd., London, 1960.

Treatise on Powder Metallurgy, vol. 1, C. G. Goetzel, p. 1, JntersciencePublishers, Inc., New York, 1949.

CARL D. QUARFORTH, Primary Examiner R. E. SCHAFER, Assistant ExaminerUS. Cl. X.R.

